1. Field of the Invention
The present invention relates to an apparatus for on-line cleaning a wafer chuck of a stepper system, and more particularly, to an apparatus for on-line cleaning a wafer chuck of a stepper system by using laser.
2. Description of the Prior Art
In integrated circuit processes, a lithography process is an essential technique. A lithography process is used to form a design pattern, such as a circuit pattern or a layout of an implantation area and the like, onto one or more masks. The pattern on the mask is then transferred to a semiconductor wafer by an exposure process using a stepper and a scanner. The lithography technique commonly used is a photolithography technique. The light source includes krypton fluoride (KrF) laser (248 nm), ArF laser (193 nm), F2 laser (157 nm), and so forth, wherein the KrF laser is more popular. Other exposure techniques and non-optical lithography techniques, such as extreme UV (EUV) and e-beam, still remain to be developed.
Please refer to FIG. 1 of a schematic diagram of a stepper system 10 for an exposure process according to the prior art. As shown in FIG. 1, the stepper system 10 comprises a wafer stage 12 capable of horizontally moving along x-y directions and vertically moving along a z direction, a wafer chuck 14 fixed on the wafer stage 12 utilizing vacuuming to hold a semiconductor wafer thereon, an excimer laser generation module 16 for generating a pulse laser, a laser beam delivery system 18 for guiding the pulse laser, and a projection lens system 20 for focusing the pulse laser.
During the exposure process, the excimer laser generation module 16 generates a KrF pulse laser with a predetermined wavelength, and then the laser beam delivery system 18 guides the pulse laser to the wafer chuck 14 on the wafer stage 12 at a predetermined incident angle. By using the projection lens system 20, the pulse laser is focused onto the semiconductor wafer (not shown) on the wafer chuck 14. Then the semiconductor wafer on the wafer chuck 14 is carried by the movement of the wafer stage 12, and the pulse laser is applied to a portion of a surface of the semiconductor wafer through a mask and projection lenses thereafter. Finally, the pattern on the mask is transferred to the whole wafer surface by utilizing the wafer stage 12 of the stepper system 10 steppedly.
Generally, the wafer chuck 14 fixed on the wafer stage 12 is employed as a carrier to carry and handle the wafer during the exposure process. Since the wafer chuck 14 utilizes vacuuming to hold the wafer thereon, the surface of wafer chuck 14 directly contacts with the back of wafer. Thus, the wafer chuck 14 is easily contaminated and the flatness and roughness of its surface influence the quality of the exposed pattern on the wafer. Unfortunately, since the back surface of the semiconductor wafer is frequently contaminated by organic materials during the coating or the baking process, the surface flatness of the wafer chuck 14 is thus defective due to contaminants remained on the wafer chuck 14, causing surfaces of next batches of wafers tilted for the exposure process and leading to defective pattern transfer thereon. Moreover, the contaminants on the wafer chuck surface cause orderly or random local defocuses as well and thus affect both the quality and the effect of the exposure process.
To ensure the quality of the exposure process and the yield of the products, ultra-flat monitor wafers and a machine detecting system are employed in most alignment systems to daily monitor the surface flatness of the wafer chuck and detect contaminants on the wafer chuck surface so as to reflect the local defocuses thereon. Once the contaminants are detected, a cleaning process is performed manually by using a grindstone, a wiper or chemicals, such as isopropyl alcohol (IPA), to remove the contaminants from the surface of the wafer chuck. However, the wafer chuck is hardly cleaned by this cleaning method due to many crannies in the wafer chuck surface. In addition, the complicated alignment system around the wafer chuck is frequently touched during the cleaning process, thus leading to malfunctions of the alignment system. An alternative method to completely remove the contaminants on the wafer chuck is to unload the wafer chuck for the cleaning process. However, this method requires certain times of adjustment after the wafer chuck is reloaded again, and thus costs lots of man-hours and a long down time of the machine. Besides, when the wafer chuck is unloaded, external contaminants can easily contaminate the stepper system due to a cover thereof opened.
It is therefore a primary objective of the present invention to provide a stepper system with an apparatus for on-line cleaning a wafer chuck with laser, so as to solve the previously mentioned problems.
According to the claimed invention, the stepper system, having an apparatus for on-line cleaning the wafer chuck with laser, comprises a wafer stage capable of horizontally moving along x-y directions, a wafer chuck fixed on the wafer stage and having a top surface contaminated by organic materials, a laminar flow chamber used to isolate the wafer chuck during the cleaning of the wafer chuck, an excimer laser generation module, a laser beam delivery system and a projection lens system both for guiding the pulse laser to the contaminated top surface of the wafer chuck, and decomposing the organic materials thereon by the pulse laser, and an inert gas supply for providing an inert gas flow to remove the decomposed organic materials from the top surface of the wafer chuck. The stepper system further comprises a control system capable of being combined with an original control device and used to control the global or local cleaning of the contaminated top surface of the wafer chuck.
It is an advantage of the present invention that a laser cleaning process is employed to remove contaminants on the surface of the wafer chuck and thus avoids the various shortcomings of a manually cleaning process. In addition, the present invention further combines a stepper system with a control system to carry out an on-line cleaning process for the wafer chuck. Efforts spent on handling abnormal situations during flatness monitor process of the wafer chuck are saved, the down time of the stepper system is reduced, and contamination to the system environment is prevented therefore. Additionally, both the exposure process and the cleaning process for the wafer chuck use a same light source. The manufacturing cost is thus reduced with an acceptable performance of the exposure process ensured.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.